1. Field of the Invention
The present invention relates to an amplifier capable of using a power supply voltage higher than all of its process voltages.
2. Related Art
Modern CMOS processes are designed to operate with a certain supply voltage. If a voltage difference greater than this voltage is applied across the terminals of a transistor during operation, serious reliability problems may arise. Specifically, after repeated instances of exposure that exceeds the maximum breakdown voltage, the gate oxide of the transistor may be significantly weakened and eventually be punctured, thereby resulting in device failure.
Usually processes provide multiple “flavors” of devices, i.e. transistors that have different characteristics and therefore can sustain different voltages. For example, in an exemplary 0.18μ CMOS process, core devices are formed with a thin oxide and operate with a supply voltage of 1.8 V, whereas input/output (I/O) devices are formed with thick oxide and therefore may operate with a supply voltage of 3.3 V.
Unfortunately, integrated circuits designed for portable applications (e.g. laptops, PDAs, etc.) often operate with a battery whose power supply voltage is typically higher than 3.3 V. For example, a typical power supply voltage for a fully charged cell phone battery may be 4.2 V. In some systems, external regulators may be used to convert the battery voltage to lower voltages that may be safely used by I/O and core devices (e.g. 3.3 V and 1.8V, respectively). However, it is often desirable to interface on chip with the battery voltage, either to take advantage of the available higher power supply voltage, or to implement the supply regulators internally to the chip.